13,526 research outputs found
A wave function based ab initio non-equilibrium Green's function approach to charge transport
We present a novel ab initio non-equilibrium approach to calculate the
current across a molecular junction. The method rests on a wave function based
description of the central region of the junction combined with a tight binding
approximation for the electrodes in the frame of the Keldysh Green's function
formalism. In addition we present an extension so as to include effects of the
two-particle propagator. Our procedure is demonstrated for a dithiolbenzene
molecule between silver electrodes. The full current-voltage characteristic is
calculated. Specific conclusions for the contribution of correlation and
two-particle effects are derived. The latter are found to contribute about 5%
to the current. The order of magnitude of the current coincides with
experiments.Comment: 21 pages, 3 figure
Civil Procedure-Title 8.01: Virginia\u27s New Civil Procedure Act
On October 1, 1977, Title 8 of the Code of Virginia was repealed and Title 8.01 became effective. The revisers of Title 8 have produced an extensive, as well as comprehensive, change in the statutes which govern civil procedure in Virginia. Most of the provisions have been rewritten, deleted or moved to other titles. With several notable exceptions, civil procedure in Virginia will remain basically unchanged. Much of the revisers work leaves Title 8 substantively intact. The major changes will be discussed in a chapter by chapter analysis of Title 8.01 in Section II of this article
High resolution Ge/Li/ spectrometer reduces rate-dependent distortions at high counting rates
Modified spectrometer system with a low-noise preamplifier reduces rate-dependent distortions at high counting rates, 25,000 counts per second. Pole-zero cancellation minimizes pulse undershoots due to multiple time constants, baseline restoration improves resolution and prevents spectral shifts
Damped finite-time-singularity driven by noise
We consider the combined influence of linear damping and noise on a dynamical
finite-time-singularity model for a single degree of freedom. We find that the
noise effectively resolves the finite-time-singularity and replaces it by a
first-passage-time or absorbing state distribution with a peak at the
singularity and a long time tail. The damping introduces a characteristic
cross-over time. In the early time regime the probability distribution and
first-passage-time distribution show a power law behavior with scaling exponent
depending on the ratio of the non linear coupling strength to the noise
strength. In the late time regime the behavior is controlled by the damping.
The study might be of relevance in the context of hydrodynamics on a nanometer
scale, in material physics, and in biophysics.Comment: 9 pages, 4 eps-figures, revtex4 fil
Arrested phase separation in reproducing bacteria: a generic route to pattern formation?
We present a generic mechanism by which reproducing microorganisms, with a
diffusivity that depends on the local population density, can form stable
patterns. It is known that a decrease of swimming speed with density can
promote separation into bulk phases of two coexisting densities; this is
opposed by the logistic law for birth and death which allows only a single
uniform density to be stable. The result of this contest is an arrested
nonequilibrium phase separation in which dense droplets or rings become
separated by less dense regions, with a characteristic steady-state length
scale. Cell division mainly occurs in the dilute regions and cell death in the
dense ones, with a continuous flux between these sustained by the diffusivity
gradient. We formulate a mathematical model of this in a case involving
run-and-tumble bacteria, and make connections with a wider class of mechanisms
for density-dependent motility. No chemotaxis is assumed in the model, yet it
predicts the formation of patterns strikingly similar to those believed to
result from chemotactic behavior
Statistical properties of a free-electron laser revealed by the Hanbury Brown and Twiss interferometry
We present a comprehensive experimental analysis of statistical properties of
the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH
at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry.
The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and
20.8 nm. We determined the 2-nd order intensity correlation function for all
wavelengths and different operation conditions of FLASH. In all experiments a
high degree of spatial coherence (above 50%) was obtained. Our analysis
performed in spatial and spectral domains provided us with the independent
measurements of an average pulse duration of the FEL that were below 60 fs. To
explain complicated behaviour of the 2-nd order intensity correlation function
we developed advanced theoretical model that includes the presence of multiple
beams and external positional jitter of the FEL pulses. By this analysis we
determined that in most experiments several beams were present in radiating
field and in one of the experiments external positional jitter was about 25% of
the beam size. We envision that methods developed in our study will be used
widely for analysis and diagnostics of the FEL radiation.Comment: 29 pages, 14 figures, 3 table
Feasibility studies of a Level-1 Tracking Trigger for ATLAS
The existing ATLAS Level-1 trigger system is seriously challenged at the SLHC's higher luminosity. A hardware tracking trigger might be needed, but requires a detailed understanding of the detector. Simulation of high pile-up events, with various data-reduction techniques applied will be described. Two scenarios are envisaged: (a) regional readout - calorimeter and muon triggers are used to identify portions of the tracker; and (b) track-stub finding using special trigger layers. A proposed hardware system, including data reduction on the front-end ASICs, readout within a super-module and integrating regional triggering into all levels of the readout system, will be discussed
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